Among various known types of disk drive devices, those using hard type magnetic disks are widely used when a small dimension and a lot of information are required. Such a magnetic disk driving device has a speed of high revolution of a magnetic disk and a high density of information recording. Therefore, improved velocity and accuracy of the seeking motion (a motion of a head to go to a track to detect information regarding its own proper position to be taken in another track) are very important. The seeking is effected, using as a reference the zero track provided at the outermost circumference in a data zone on a magnetic disk. In this connection, detection of the zero track position must be accurate particularly.
A prior art arrangement of zero track position detecting mechanism includes a shutter which is formed on a carriage supporting a magnetic head thereon, and includes a photo interrupter which produces a detection signal when a light path is blocked by the shutter when the magnetic head approaches the zero track. A logical multiplication is obtained between the detection signal from the photo interrupter and a driving signal which excites a predetermined phase of a stepping motor for driving the carriage so as to cause a zero track signal to be produced, which signal indicates that the magnetic head is properly located at the zero track.
The mechanical detecting means described above requires the photo interrupter or other mechanical parts which prevent a desired reduction in size and thickness of the magnetic disk driving device, and cannot perform an expected operation in an increased recording density due to its limited accuracy of the zero track detection which depends on the resolving power of the photo interrupter.
To overcome these problems, one of the present inventors formerly proposed a zero track restoring method in which zero track detection is performed by an electrical detection means. According to the electrical detecting means, a servo zone provided on a magnetic disk in the form of a radially extending band is divided into two regions one of which is a first region in the rotationally upstream half and the other is a second region in the rotationally downstream half. In the data zone, servo information is written alternatingly in the first and second regions of the servo zone, whereas in the outer guard band, servo information is written in one of the first and second regions. Partition between the outer guard band and the data zone is roughly detected, based on a difference between the locations of the written servo information, and a driving signal for exciting a predetermined phase of the stepping motor is supplied to locate the magnetic head at a precise position on the zero track. That is, the zero track restoration is established.
Certainly, the aforegoing electrical detecting means does not require any mechanical part such as photo interrupter, facilitates a significant reduction of the size and thickness of the magnetic disk driving device, and has an improved resolving power of the detecting means.
Since the track position of the magnetic head is not known clearly when the power source of the magnetic disk drive device is switched on, zero track restoration is effected so as to meet a track number of a track counter with the track position of the magnetic head by once moving back the magnetic head to the zero track and resetting the track counter in a drive provided for controlling the position of the magnetic head. Zero track restoration is also effected when the magnetic head is moved by an external force and causes a seeking error in which the track position of the magnetic head does not coincide with the track number of the track counter. Further, zero track restoration is effected in a simple seeking motion to the zero track.
In case that zero track restoration is effected to cure a seeking error, the aforegoing prior art mechanical detecting means readily performs the zero track restoration to actually locate the magnetic head at a proper position on the zero track, referring to the track number of the track counter and the detection signal from the photo interrupter. However, the zero track restoring method using the electrical detecting means proposed by the present inventors and explained above cannot refer to a data such as the detection signal of the photo interrupter of the mechanical detecting means, and therefore must simply refer to the track number of the track counter. Therefore, if the track counter demonstrates track number "zero" even though the magnetic head is not actually located at the zero track, the drive itself makes an erroneous judgement that the zero track restoration is finished, resulting in detection of a zero track signal. In this case, re-correction of the head position is not effected even if a restoration signal is applied again.
A certain type of magnetic disk driving device is configured to judge presence or absence of a seeking error upon issuance of a zero track signal. In presence of any seeking error, the magnetic head is moved inwardly by an amount of several steps and subsequently moved step by step outwardly by a restoration signal in the attempt to establish zero track restoration. However, the prior art device involves a problem that zero track restoring motion is repeated as far as it simply refers to the track number of the track counter.